In recent years, there has been a trend toward increasing the inlet temperature of combustion gas flowing into gas turbine blades in order to improve gas turbine performance, and it will reach 1700° C. in future. Thus, several cooling structures for gas turbine blades have been developed. One such known cooling structures is a serpentine channel in which a plurality of cooling channels are formed within the blade along the span-wise direction, and these channels are connected at the base end or the tip end of the blade in a folded manner (see PTL 1).